Trigger controlled power operated tools, appliances, or the like are well known in the art, some of which include electrical control circuits for controlling the speed of the motor of the tool by movement of the trigger mechanism. Many of these tools, particularly electrical drills, employ a thyristor such as, for example, a silicon controlled rectifier unit (commonly called an SCR), or a triac, in their electrical control systems. In operation, such thyristors generate heat when under normal operation, such heat often causing premature failure of the units. Oftentimes the thyristors are the first part of the control system to fail, thus rendering the tool totally inoperative.
In some electrical hand tools the thyristor overheating problem has been partially alleviated by placing the thyristor directly in the exiting airstream generated by the fan which cools the motor. Although such cooling of the thyristor has reduced or prolonged their failure, the fact remains that even in tools having a fan-cooled thyristor, the thyristor is still the first part to fail. Even though a thyristor is relatively inexpensive in comparison with other parts of the tool, their replacement is most difficult if not impossible because of its inaccessability in the tool and difficulty of replacement in the control circuit even if accessability thereto is accomplished.
The SCR or triac (or like thyristor) overheating problem has also been partially alleviated by employing a control circuit bypass switch which cuts out the SCR or triac from the control circuit when the motor reaches substantially full speed, as shown and described in U.S. Pat. No. 3,536,973, issued Oct. 27, 1970, to applicant's assignee.